DRAM Layout with Vertical FETS and Method of Formation

ABSTRACT

DRAM cell arrays having a cell area of about 4F 2  comprise an array of vertical transistors with buried bit lines and vertical double gate electrodes. The buried bit lines comprise a silicide material and are provided below a surface of the substrate. The word lines are optionally formed of a silicide material and form the gate electrode of the vertical transistors. The vertical transistor may comprise sequentially formed doped polysilicon layers or doped epitaxial layers. At least one of the buried bit lines is orthogonal to at least one of the vertical gate electrodes of the vertical transistors.

FIELD OF THE INVENTION

The invention relates to improved semiconductor structures for highdensity device arrays and, in particular, to memory cell arrays andprocesses for their formation.

BACKGROUND OF THE INVENTION

There are two major types of random-access memory cells: dynamic andstatic. Dynamic random-access memories (DRAMs) can be programmed tostore a voltage which represents one of two binary values, but requireperiodic reprogramming or “refreshing” to maintain this voltage for morethan very short time periods. Static random-access memories are named“static” because they do not require periodic refreshing.

DRAM memory circuits are manufactured by replicating millions ofidentical circuit elements, known as DRAM cells, on dies on a singlesemiconductor wafer. Each DRAM cell is an addressable location that canstore one bit (binary digit) of data. In its most common form, a DRAMcell consists of two circuit components: a field effect transistor (FET)and a capacitor.

FIG. 1 illustrates a portion of an exemplary DRAM memory circuitcontaining two neighboring DRAM cells 42. For each cell, capacitor 44has two connections, located on opposite sides of the capacitor 44. Thefirst connection is to a reference voltage, which is typically one halfof the internal operating voltage (the voltage corresponding to onelogical state) of the circuit. The second connection is to the drain ofthe FET 46. The gate of the PET 46 is connected to the word line 48, andthe source of the FET is connected to the bit line 50. This connectionenables the word line 48 to control access to the capacitor 44 byallowing or preventing a signal (a logic “0” or a logic “1”) on the bitline 50 to be written to, or read from, the capacitor 44. In somearrangements, the body of the FET 46 is connected to body line 76, whichis used to apply a fixed potential to the semiconductor body.

The manufacturing of a DRAM cell typically includes the fabrication of atransistor, a capacitor, and three contacts: one each to the bit line,the word line, and the reference voltage Vr. As DRAM manufacturing is ahighly competitive business, there is continuous pressure to decreasethe size of individual cells and to increase memory cell density toallow more memory to be squeezed onto a single memory chip, especiallyfor densities greater than 256 Megabits. Limitations on cell sizereduction include the passage of both active and passive word linesthrough the cell, the size of the cell capacitor, and the compatibilityof array devices with non-array devices.

Conventional folded bit line cells of the 256 Mbit generation withplanar devices have a size of at least 8F², where F is the minimumlithographic feature size. If a folded bit line is not used, the cellmay be reduced to 6 or 7F². To achieve a smaller size, vertical devicescould be used. In this manner, cell sizes of 4F² may be achieved byusing vertical transistors stacked either below or above the cellcapacitors, as in the “cross-point cell” of W. F. Richardson et al., ATrench Transistor Cross-Point DRAM Cell, IEDM Technical Digest, pp.714-17 (1985). Known cross-point cells, which have a memory cell locatedat the intersection of each bit line and each word line, are expensiveand difficult to fabricate because the structure of the array devices istypically incompatible with that of non-array devices. Other knownvertical cell DRAMs using stacked capacitors have integration problemsdue to the extreme topography of the capacitors.

There is needed, therefore, a DRAM cell having an area of about 4F² thatachieves high array density while maintaining structural commonalitybetween array and peripheral (non-array) features. Also needed aresimple methods of fabricating a DRAM cell that maximizes common processsteps during the formation of array and peripheral devices.

Additional advantages and features of the present invention will beapparent from the following detailed description and drawings whichillustrate preferred embodiments of the invention.

SUMMARY OF THE INVENTION

Exemplary embodiments of the present invention provide a DRAM cell arraycapable of achieving a cell area of about 4F² which comprises an arrayof vertical transistors with at least one buried bit line and at leastone vertical gate electrode. The buried bit line and the vertical gateelectrode of the vertical transistors are substantially orthogonal. Alsoprovided are processes for fabricating DRAM cell arrays with verticalFET transistors having buried bit lines and vertical gate electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a known DRAM cell.

FIG. 2 is a cross-sectional view of a SOI substrate undergoing initialstages of a process according to a first embodiment of the presentinvention.

FIG. 3 shows the SOI substrate of FIG. 2 at a processing step subsequentto that shown in FIG. 2.

FIG. 4 shows the SOI substrate of FIG. 2 at a processing step subsequentto that shown in FIG. 3.

FIG. 5 shows the SOI substrate of FIG. 2 at a processing step subsequentto that shown in FIG. 4.

FIG. 6 shows the SOI substrate of FIG. 2 at a processing step subsequentto that shown in FIG. 5.

FIG. 7 shows two side-to-side cross-sectional views (A-A and B-B) of aSOI substrate of FIG. 11 undergoing the process according to the firstembodiment of the present invention and at a processing step subsequentto that shown in FIG. 6.

FIG. 8 shows the SOI substrate of FIG. 7 at a processing step subsequentto that shown in FIG. 7.

FIG. 9 shows the SOI substrate of FIG. 7 at a processing step subsequentto that shown in FIG. 8.

FIG. 10 shows the SOI substrate of FIG. 7 at a processing stepsubsequent to that shown in FIG. 9.

FIG. 11 is a perspective view of a memory array fabricated according toa first embodiment of the present invention.

FIG. 12 is a cross-sectional view of a wafer substrate undergoing theprocess according to a second embodiment of the present invention.

FIG. 13 shows the wafer of FIG. 12 at a processing step subsequent tothat shown in FIG. 12.

FIG. 14 shows the wafer of FIG. 12 at a processing step subsequent tothat shown in FIG. 13.

FIG. 15 shows the wafer of FIG. 12 at a processing step subsequent tothat shown in FIG. 14.

FIG. 16 shows the wafer of FIG. 12 at a processing step subsequent tothat shown in FIG. 15.

FIG. 17 shows two side-to-side cross-sectional views (A-A and B-B) of asemiconductor wafer of FIG. 18 undergoing the process according to thesecond embodiment of the present invention and at a processing stepsubsequent to that shown in FIG. 16.

FIG. 18 is a perspective view of a memory array fabricated according toa second embodiment of the present invention.

FIG. 19 is a perspective view of a memory array fabricated according toa third embodiment of the present invention.

FIG. 20 is a cross-sectional view of a SOI substrate undergoing theprocess according to a fourth embodiment of the present invention.

FIG. 21 shows the SOI substrate of FIG. 20 at a processing stepsubsequent to that shown in FIG. 20.

FIG. 22 shows the SOI substrate of FIG. 20 at a processing stepsubsequent to that shown in FIG. 21.

FIG. 23 shows the SOI substrate of FIG. 20 at a processing stepsubsequent to that shown in FIG. 22.

FIG. 24 shows the SOI substrate of FIG. 20 at a processing stepsubsequent to that shown in FIG. 23.

FIG. 25 shows two side-to-side cross-sectional views (A-A and B-B) of aSOI substrate of FIG. 29 undergoing the process according to the fourthembodiment of the present invention and at a processing step subsequentto that shown in FIG. 24.

FIG. 26 shows the cross-sectional views of the SOI substrate of FIG. 25at a processing step subsequent to that shown in FIG. 25.

FIG. 27 shows the cross-sectional views of the SOI substrate of FIG. 25at a processing step subsequent to that shown in FIG. 26.

FIG. 28 shows the cross-sectional views of the SOI substrate of FIG. 25at a processing step subsequent to that shown in FIG. 27.

FIG. 29 is a perspective view of a memory array fabricated according toa fourth embodiment of the present invention.

FIG. 30 is a cross-sectional view of a wafer substrate undergoing theprocess according to a fifth embodiment of the present invention.

FIG. 31 shows the wafer of FIG. 30 at a processing step subsequent tothat shown in FIG. 30.

FIG. 32 shows the wafer of FIG. 30 at a processing step subsequent tothat shown in FIG. 31.

FIG. 33 shows the wafer of FIG. 30 at a processing step subsequent tothat shown in FIG. 32.

FIG. 34 shows the wafer of FIG. 30 at a processing step subsequent tothat shown in FIG. 33.

FIG. 35 shows two side-to-side cross-sectional views (A-A and B-B) of asemiconductor wafer of FIG. 36 undergoing the process according to thesecond embodiment of the present invention and at a processing stepsubsequent to that shown in FIG. 35.

FIG. 36 is a perspective view of a memory array fabricated according toa fifth embodiment of the present invention.

FIG. 37 is a perspective view of a memory array fabricated according toa sixth embodiment of the present invention.

FIG. 38 is a partial top view of a square layout of the memory array ofFIG. 36.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to variousspecific embodiments in which the invention may be practiced. Theseembodiments are described with sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatother embodiments may be employed, and that structural and logicalchanges may be made without departing from the spirit or scope of thepresent invention.

The term “substrate” in the following description may include anysemiconductor-based structure on or at the surface of which circuitrymay be formed. The structure should be understood to include silicon,silicon-on insulator (SOI), silicon-on-sapphire (SOS), doped and undopedsemiconductors, epitaxial layers of silicon supported by a basesemiconductor foundation, and other semiconductor structures. Thesemiconductor need not be silicon-based. The semiconductor could besilicon-germanium, germanium, or gallium arsenide. When reference ismade to the substrate in the following description, previous processsteps may have been utilized to form regions or junctions on or over thebase semiconductor or foundation.

Referring now to the drawings, where like elements are designated bylike reference numerals, a portion of an embodiment of device array 100of the present invention is illustrated in FIG. 11. The device array 100comprises a plurality of DRAM cells 99 formed over or within asubstrate, for example over the illustrated SOI substrate 110. Each DRAMcell 99 comprises two devices, a vertical transistor 90 and a capacitor80 located above the vertical transistor 90 having one electrodeelectrically connected to transistor 90.

The vertical transistor 90 comprises a vertical stack of three dopedsilicon layers 12, 14 and 16 over an insulation layer 6. An exemplaryN-channel transistor 90, as illustrated in FIG. 11, would be formedusing a SOI substrate 110 of a first conductivity type, for examplep-type, a source 12 of a second conductivity type (n+), a lightly-dopedbody region 14 of the first conductivity type, and a drain 16 of asecond conductivity type (n+). If P-channel devices were desired, thedoping types and doping levels of these elements would be adjustedaccordingly, as known in the art.

The vertical transistor 90 is a MOSFET (metal-oxide-semiconductor FET)device having doped region 12 coupled to a pair of buried bit lines (BL)25. The drain 16 of the transistor 90 is in contact with one electrodeof capacitor 80. The buried bit lines 25, preferably of a silicidematerial, are formed so that they contact the source 12 of eachtransistor 90 of a particular column in the array 100. A pair of activeword lines 35 of a conductive material, such as doped polysilicon, of asecond conductivity type (n+), is formed to act as the gate of eachtransistor 90, and to electrically connect all of the cells 99 of agiven row in the array 100.

The device array 100 is manufactured through a process described asfollows and illustrated in FIGS. 2-10. First, a SOI substrate 110 isformed by known methods in the art, for example, separation by implantedoxygen (SIMOX), bonding and etching back (BESOI), and zone melting andrecrystallization (ZMR), among others. According to the bonding andetching back method, the process for the formation of the SOI substrate110 starts with the preparation of a silicon substrate 4 (FIG. 2). Thesilicon substrate 4 is thermally oxidized to grow a layer of siliconoxide 6 (FIG. 2) with a thickness of about 1 micron. Subsequently, asingle crystalline silicon substrate 8 is opposed to the silicon oxidelayer 6, as also shown in FIG. 2. In this manner, the silicon substrate4, with the oxide layer 6, is contacted with the crystalline siliconsubstrate 8, and the resultant structure is heated to a temperature ofabout 1000° C. so that the crystalline silicon substrate 8 adheres tothe silicon oxide layer 6. Thus, a resultant SOI substrate 110 (FIG. 2)is formed of the silicon substrate 4, the silicon oxide layer 6, and thecrystalline silicon substrate 8.

Subsequent to the formation of the SOI substrate 110, device layers 12,14, 16 are next formed by implant doping of the crystalline siliconsubstrate 8 appropriately to form n+, p−, n+ regions or layers 12, 14,16, as also shown in FIG. 2. Thus, the first device layer 12 (FIG. 2) ispreferably a doped silicon layer of a second conductivity type (n+)approximately 0.4 microns thick, formed by implanting n-type dopantssuch as phosphorous (P), arsenic (As) or antimony (Sb) into crystallinesilicon substrate 8 to form the n+ silicon layer 12. A heat treatment,such as an anneal treatment at about 600° C. to about 1000° C., may beoptionally employed to activate the dopant within the n+ silicon layer12. The second device layer 14 is preferably a lightly-doped siliconlayer of a first conductivity type (p−) and its thickness can be variedfor desired channel lengths (e.g., about 0.05 to about 0.5 microns). Thethird device layer 16 is also preferably a doped silicon layer of asecond conductivity type (n+) about 0.2 microns thick. A heat treatmentmay be also optionally employed to activate the dopants within the p−silicon layer 14 and the n+ silicon layer 16.

As also shown in FIG. 2, an insulating layer 18, preferably formed of anitride or oxide material, is formed on top of the third device layer 16by a deposition method or other suitable methods. The insulating layer18 may be also formed of silicon dielectrics such as silicon nitride orsilicon oxide, but TEOS or carbides may be used also. Preferably,insulating layer 18 comprises a nitride material formed via CVD, PECVDand LPCVD deposition procedures, for example, at a temperature betweenabout 300° C. to about 1000° C., to a thickness of about 500 Angstromsto about 2,000 Angstroms.

A photoresist and mask are then applied over the insulating layer 18,and photolithographic techniques are used to define a set of parallelrows on the array surface. A directional etching process such as plasmaetching or reactive ion etching (RIE) is used to etch through theinsulating layer 18 and through the device layers 16, 14 and into devicelayer 12 to form a first set of trenches 21, as depicted in FIG. 3.Preferably, the first set of trenches 21 extends into the first devicelayer 12 about 1,000 Angstroms.

After removal of the resist, a nitride film 22 is formed on the sides ofthe first set of trenches 21 by depositing a layer of CVD nitride, forexample, and directionally etching to remove excess nitride fromhorizontal surfaces. The nitride film 22 (FIG. 3), which is about 100Angstroms thick, acts as an oxidation and etching barrier duringsubsequent steps in the fabrication process. Anisotropic etching such asRIE is subsequently conducted to deepen the first set of trenches 21 byabout an additional 0.3 microns and to remove, therefore, the remainderof the n+ silicon layer 12.

Next, as shown in FIG. 4, a conductive layer 24 of a metal capable offorming a silicide is formed over gate stacks 20, over the nitridespacers 22 and within the first set of trenches 21 by RF or DCsputtering, or by other similar methods such as CVD, to a thickness ofabout 100 Angstroms to about 800 Angstroms. Subsequent to the depositionof the metal capable of forming a silicide, the substrate is subjectedto a rapid thermal anneal (RTA), typically for about 10 to 60 seconds,using a nitrogen ambient at about 600° C. to about 850° C., so that themetal in direct contact with the doped silicon layer 12 is converted toits silicide and forms buried silicide regions 25 (which are the buriedbit lines 25 of the device array 100 of FIG. 11). Preferably, the metalcapable of forming a silicide is a combination of cobalt/titaniumnitride material that forms cobalt silicide bit line 25. However, themetal silicide may comprise any metal capable of forming a silicide,including but not limiting to cobalt, nickel, molybdenum, titanium,tungsten, tantalum, and platinum, among others, and combinations of suchmaterials. In addition, the metal silicide may also comprisecombinations of silicides doped with nitrogen, such as cobalt nitridesilicide, tungsten nitride silicide, or a combination of tungstennitride silicide/tungsten silicide, for example.

Subsequent to the formation of buried silicide bit lines 25, theunreacted metal is stripped, together with the protective nitridespacers 22 (FIG. 5) and the insulating layer 18 (FIG. 5), and nitridematerial 26 is formed within the first set of trenches 21 (FIG. 6).Although nitride material 26 is preferred, the invention alsocontemplates the formation of an oxide, such as silicon oxide forexample, to fill in the first set of trenches 21. The device array 100is then planarized by any suitable means, such as chemical mechanicalpolishing (CMP), stopping at the third device layer 16.

Reference is now made to FIGS. 7-10 which schematically illustrate theformation of word lines 35 (FIG. 10) of the vertical transistors 90(FIG. 11). For a better understanding of the formation of word lines 35,FIGS. 7-10 are illustrated as side-to-side cross-sectional views of thedevice array 100 of FIG. 11, taken along lines A-A and B-B and at aninitial stage of processing, but subsequent to the formation of thesilicide bit lines 25 described above. The illustrations in FIGS. 7-10are cross-sectional views taken normal to the buried bit lines 25 (A-A)but at two different locations at the array 100 (A-A) and (B-B).

FIG. 7 (A-A) illustrates stack 20 after the formation of the silicidebit lines 25 of FIG. 6 and after the formation of a second set oftrenches 23. The formation of the second set of trenches 23 is similarto the formation of the first set of trenches 21 (FIG. 3). Accordingly,a photoresist and mask are applied over the third device layer 16, andphotolithographic techniques are used to define a set of parallelcolumns on the array surface. A directional etching process such asplasma etching or reactive ion etching (RIE) is used to etch through thedevice layers 16, 14 and into device layer 12, as well as layer 26 atapproximately equivalent etch rates, to form the second set of trenches23, as depicted in FIG. 7.

FIG. 8 illustrates the next step in the process, in which the second setof trenches 23 (FIG. 7) are filled with an insulating material 31,preferably an oxide material such as silicon oxide, which is etched backby known methods in the art to form oxide layer 32, as shown in FIG. 9.The height of the oxide layer 32 is tailored to allow isolation of thealready-formed silicide bit lines 25 from the to-be-formed word lines orgate electrodes 35. Subsequent to the formation of the oxide layer 32, athin gate oxide layer 34 and a gate electrode 35 are sequentially formedon the sidewalls of the stacks 20, as shown in FIG. 10. The thin gateoxide layer 34, which will act as a gate insulator layer, may comprisesilicon dioxide (SiO₂), for example, which may be thermally grown in anoxygen ambient, at a temperature between about 600° C. to about 1000° C.and to a thickness of about 10 Angstroms to about 100 Angstroms. Thegate insulator is not limited to silicon oxide and other dielectricmaterials such as oxynitride, Al₂O₃, Ta₂O₅ or other high k material maybe used as gate insulator layer.

As illustrated in FIG. 10, a gate layer 35 is formed over the thin gateoxide layer 34. According to an embodiment of the present invention, thegate layer 35 is formed of doped polysilicon, for example, which may bedeposited over the thin gate oxide layer 34 by, for example, a lowplasma chemical vapor deposition (LPCVD) method at a temperature ofabout 300° C. to about 700° C. and to a thickness of about 100 Angstromsto about 2,000 Angstroms. Anisotropic plasma etching is conducted todefine the gate electrode 35 orthogonal to the buried silicide bit lines25. Subsequent processing steps are then applied to complete theformation of the device array 100 comprising MOSFET transistors 90. Eachof the vertical transistor 90 of a particular column in the array 100 isformed of drain 16 and source 12, with double vertical gate electrode 35formed over the thin gate oxide 34 of each transistor 90. The gateelectrode is vertical and orthogonal to the buried bit line 25. Thevertical gate electrode forms word line 35 which electrically connectsall of the cells 99 of a given row in the array 100.

Conventional processing methods may then be used to form contacts andwirings to connect the device array to peripheral circuits, and to formother connections. For example, the entire surface may be covered with apassivation layer of, for example, silicon dioxide, BSG, PSG, or BPSG,which is CMP planarized and etched to provide capacitor trenches overthe transistors 90, in which capacitors 80 are formed, as well ascontact holes which may then be metallized to interconnect the wordlines, bit lines and capacitors 80 of the memory cells 99 into anoperative memory array. Conventional multiple layers of conductors andinsulators may also be used to interconnect the structures.

Reference is now made to FIGS. 12-18 which illustrate the formation ofdevice array 200 (FIG. 18) in accordance with a second embodiment of thepresent invention. In this embodiment, the device array 200 comprises aplurality of DRAM cells 299 formed over or within a p-type wafersubstrate 210, and not within a SOI substrate, such as the SOI substrate110 of the previously-described embodiment. As in the first embodiment,and as shown in FIG. 18, each DRAM cell 299 comprises two devices, avertical transistor 290 and a capacitor 80 located above the transistor290. The transistor 290 is formed of a vertical stack of three dopedsilicon layers formed by appropriately doping the p-type wafer substrate210. An exemplary n-channel device, as illustrated in FIG. 18, would beformed using a wafer substrate 210 of a first conductivity type, forexample p+, a source 12 of a second conductivity type (n+), alightly-doped body region 14 of a first conductivity type (p−), and adrain 16 of a second conductivity type (n+). If p-channel devices weredesired, the doping types and levels of these elements would be adjustedaccordingly, as known in the art. The drain 16 of the transistor 290 isin contact with one electrode of capacitor 80. Buried silicide bit line25 is formed of a conductive silicide and contacts the source 12 of eachtransistor 290 of a particular column in the array 200. Active word lineor gate electrode 35 is formed of a conductive material such as dopedpolysilicon of a second conductivity type (n+) and acts as the gate ofeach transistor 290 to electrically connect all of the cells 299 of agiven row in the array 200.

FIGS. 12-17 illustrate the processing steps for the formation of thedevice array 200 manufactured in accordance with a second embodiment ofthe present invention. As noted above, these processing steps aresimilar in part with the processing steps of FIGS. 2-10 but differ inthat the SOI substrate 110 of the first embodiment is replaced with ap-type substrate 210 in the second embodiment. Accordingly, forexemplary purposes only, the substrate 210 will be described as asilicon substrate, and the following process should be modified asappropriate and as known in the art if a non-silicon substrate is used.The substrate 210 may be doped or undoped, but a p-type doped substrateis preferred. For example, substrate 210 may be a doped siliconsubstrate having an active dopant concentration within the range ofabout 1×10¹⁶ to 1×10¹⁸ atoms per cm³, more preferably about 5×10¹⁶ to5×10¹⁷ atoms per cm³.

Device layers 12, 14, 16 are next formed by doping or implanting areasof the p-type substrate 210 appropriately with p-type or n-type dopantsto form n+, p−, n+ regions or layers 12, 14, 16. In this manner, thefirst device layer 12 is preferably formed as a doped silicon layer of asecond conductivity type (n+) and about 0.4 microns thick, by implantingn-type dopants such as phosphorous (P), arsenic (As) or antimony (Sb)into p-type substrate 210 to form the n+ silicon layer 12. The seconddevice layer 14 is preferably formed as a lightly-doped silicon layer ofa first conductivity type (p−) and has a thickness that can be variedfor desired channel length (e.g., about 0.05 to about 0.5 microns). Thethird device layer 16 is preferably formed as a doped silicon layer of asecond conductivity type (n+) about 0.2 microns thick. A heat treatment,such as an anneal treatment at about 600° C. to about 1000° C., may beoptionally used to activate the dopants within the n+ silicon layer 12,the p− silicon layer 14 and the n+ silicon layer 16. The remainingundoped portion of the substrate 210 is illustrated in FIG. 12 assubstrate 210 a.

As also shown in FIG. 12, an insulating layer 18, preferably formed of anitride or oxide material, is formed on top of the third device layer 16by chemical vapor deposition (CVD) or other suitable methods. Aphotoresist and mask are then applied over the first insulating layer18, and photolithographic techniques are used to define a set ofparallel rows on the array surface. A directional etching process suchas plasma etching or reactive ion etching (RIE) is used to etch throughthe insulating layer 18 and through the device layers 14, 16 and intodevice layer 12 of the substrate 210 to form a first set of trenches 21,as depicted in FIG. 12. Preferably, the first set of trenches 21 extendsinto the first device layer 12 about 1,000 Angstroms.

After removal of the resist, a nitride film 22 (FIG. 12) is formed onthe sides of the first set of trenches 21 by depositing a layer of CVDnitride, for example, and directionally etching to remove excess nitridefrom horizontal surfaces. The nitride film 22, which is about 100Angstroms thick, acts as an oxidation and etching barrier duringsubsequent steps in the fabrication process. Anisotropic etching such asRIE is subsequently conducted to deepen the first set of trenches 21 byabout an additional 0.3 microns and to remove, therefore, the remainderof the n+ silicon layer 12.

A conductive layer 24 (FIG. 13) of a metal capable of forming a silicideis next formed over gate stacks 120, over the nitride spacers 22 andwithin the first set of trenches 21 (FIG. 13). The conductive layer 24may be formed by RF or DC sputtering, or by other similar methods suchas CVD, to a thickness of about 100 Angstroms to about 800 Angstroms.Subsequent to the deposition of the metal capable of forming a silicide,the substrate is subjected to a rapid thermal anneal (RTA), typicallyfor about 10 to 60 seconds, using a nitrogen ambient at about 600° C. toabout 850° C., so that the metal in direct contact with the dopedsilicon layers 12 is converted to its silicide and forms buried silicideregions 25 (FIG. 15, which will be the buried bit lines 25 of the devicearray 200 of FIG. 18). It must be noted, however, that no silicide formswithin the p-type substrate 210 a. Preferably, a distance “d” (FIG. 16)of about 700 Angstroms must be maintained between lower surface 25 b ofthe buried bit lines 25 and upper surface 210 b of the p-type substrate210.

As in the previously-described embodiment, the metal capable of forminga silicide may preferably be a combination of cobalt/titanium nitridematerial that forms cobalt silicide bit line 25. However, the metalsilicide may comprise any metal capable of forming a silicide, includingbut not limiting to cobalt, nickel, molybdenum, titanium, tungsten,tantalum, and platinum, among others, and combinations of suchmaterials. In addition, the metal silicide may also comprisecombinations of silicides doped with nitrogen, such as cobalt nitridesilicide, tungsten nitride silicide, or a combination of tungstennitride silicide/tungsten silicide, for example.

Subsequent to the formation of buried silicide bit lines 25, theunreacted metal is stripped, the silicide and device layer 12 is etchedstopping on the substrate 210 a together with the protective nitridespacers 22 (FIG. 15) and insulating layer 18 (FIG. 15), and a nitridematerial 26 (FIG. 16) is formed within the first set of trenches 21.Although nitride material is preferred, the invention also contemplatesthe formation of an oxide, such as silicon oxide for example, to fill inthe first set of trenches 21. The device array 200 is then planarized bychemical mechanical polishing (CMP), for example, stopping at the thirddevice layer 16.

Subsequent to the formation of the first set of trenches 21, a secondset of trenches 23 (FIG. 17) are formed by directional etching, forexample, as explained above with reference to FIGS. 7-10. The second setof trenches are then filled with an insulating material, preferably anoxide material such as silicon oxide, which is etched back by knownmethods in the art to form an oxide layer similar to oxide layer 32 ofFIG. 9. Subsequent to the formation of the oxide layer (which will beetched to form oxide regions 33 of FIG. 17), a thin gate oxide layer 34and a gate electrode 35 are sequentially formed on the sidewalls of thestacks 20, as shown in FIG. 17. The thin gate oxide layer 34 and wordlines or gate electrodes 35 (FIG. 17) of the vertical transistors 290(FIG. 18) are formed as described above with reference to the formationof the word lines or gate electrodes of the first embodiment of thepresent invention. The thin gate oxide layer 34, which will act as agate insulator layer, may comprise silicon dioxide (SiO₂), for example,which may be thermally grown in an oxygen ambient, at a temperaturebetween about 600° C. to about 1000° C. and to a thickness of about 10Angstroms to about 100 Angstroms. The gate insulator is not limited tosilicon oxide and other dielectric materials such as oxynitride, Al₂O₃,Ta₂O₅ or other high k material may be used as gate insulator layer. Gatelayer 35 may be formed of polysilicon which may be deposited over thethin gate oxide layer 34 by, for example, a low plasma chemical vapordeposition (LPCVD) method at a temperature of about 300° C. to about700° C. and to a thickness of about 100 Angstroms to about 2,000Angstroms.

Subsequent processing steps are then conducted to complete the formationof the device array 200 comprising MOSFET transistors 290. Each of thevertical transistor 290 of a particular row in the array 200 is formedof drain 16 and source 12, with gate layer 35 formed over the thin gateoxide 34 of each vertical transistor 290. The gate layer 35 is verticaland orthogonal to the buried bit line 25 formed within the p-typesubstrate 210. The vertical gate layer forms word line 35 whichelectrically connects all of the cells 299 of a given row in the array200. Capacitors are formed over the vertical transistors 290 with oneelectrode in contact with drain 16. Other processing steps are thencarried out to interconnect the word line, bit lines and capacitors ofthe memory cells 299 in a memory array, as described above withreference to the first embodiment.

FIG. 19 illustrates yet another embodiment of the present inventionaccording to which the device array 300 comprises MOSFET transistors 390which are subjected to an optional salicide process after the formationof double vertical gate electrode 335. The MOSFET transistors 390 may beformed over or within a SOI substrate (as the SOI substrate 110described above in the first embodiment and with reference to FIGS.2-11) or over or within a wafer substrate (as the p-type substrate 210described above in the second embodiment and with reference to FIGS.12-18). For illustration purposes only, the MOSFET transistors 390 arefabricated as described above within a p-type substrate 210, in a mannersimilar to that for the formation of the MOSFET transistors 290 of FIG.18. However, subsequent to the metal deposition and formation of thedevice array 200 of FIG. 18, the array is subjected to an anneal processsuch as a rapid thermal anneal (RTA) for about 10 to 60 seconds using anitrogen ambient at about 600° C. to about 850° C., to form word linesor gate electrodes 335 (FIG. 19) of metal silicides and a layer 385(FIG. 19) preferably of cobalt silicide (CoSi₂), or other silicidematerial, on top of the stack 20. The cobalt silicide formed on top ofthe stack 20 may be formed simultaneously with the formation of theburied silicide bit lines 25 or, alternatively, after the formation ofthe buried silicide bit lines 25. Of course, the metal for the formationof the silicide gate electrode 335 must be a metal which may beconverted to its silicide or a combination of such metals, for example.In this manner, the word line 335, the buried bit line 25 and the drainincluding layer 385 are all formed of a silicide material.

FIGS. 20-29 illustrate a fourth embodiment of the present invention,according to which device array 400 (FIG. 29) comprises a plurality ofDRAM cells 499, each DRAM cell 499 comprising two devices, a verticaltransistor 490 and a capacitor 80 located above the transistor 490.Vertical transistor 490 of the device array 400 comprises buried bitlines 25 disposed adjacent a gate stack comprising epitaxial layers 414,416, 418, and not doped silicon layers as in the previously-describedembodiments.

FIG. 20 illustrates a SOI substrate 110 similar to that shown in FIG. 2.As in the above-described embodiments, the SOI substrate 110 may beformed by a bonding and etching back method, according to which asilicon substrate 4 is thermally oxidized to grow a layer of siliconoxide 6 with a thickness of about 1 micron. Subsequently, an n-typesingle crystalline silicon substrate 8 is opposed to the silicon oxidelayer 6, and the silicon substrate 4, with the oxide layer 6, is thencontacted with the crystalline silicon substrate 8, and the resultantstructure is heated to a temperature of about 1000° C., so that then-type crystalline silicon of the crystalline silicon substrate 8adheres to the silicon oxide layer 6. Next, the n-type crystallinesilicon substrate 8 may be polished so that its thickness may bedecreased. Thus, the resultant SOI substrate 110 is formed of thesilicon substrate 4, the silicon oxide layer 6, and the n-typecrystalline silicon substrate 8.

Subsequent to the formation of the SOI substrate 110, a thick insulatinglayer 51, for example a thick oxide layer or a thick nitride layer ofabout 2,000 Angstroms to about 10,000 Angstroms, is formed over the SOIsubstrate 110 by chemical vapor deposition (CVD) or other suitablemethods, for example. The insulating layer 51 may be formed via PECVDand LPCVD deposition procedures, for example, at a temperature betweenabout 300° C. to about 1000° C. A photoresist and mask are then appliedover the thick insulating layer 51, and photolithographic techniques areused to define a set of parallel rows or columns 51 a on the arraysurface. A directional etching process such as plasma etching orreactive ion etching (RIE) is used to etch through the insulating layer51 and into the SOI substrate 110 to form a first set of trenches 21, asdepicted in FIG. 20. Preferably, the first set of trenches extend intothe crystalline silicon substrate 8 and stop on the upper surface of thesilicon oxide layer 6, as shown in FIG. 20.

A conductive layer 24 of a metal capable of forming a silicide is formedover insulating columns 51 a and within the first set of trenches 21(FIG. 21) by RF or DC sputtering, or by other similar methods such asCVD, to a thickness of about 100 Angstroms to about 800 Angstroms.Subsequent to the deposition of the metal capable of forming a silicide,the substrate is subjected to a rapid thermal anneal (RTA), typicallyfor about 10 to 60 seconds, using a nitrogen ambient at about 600° C. toabout 850° C., so that the metal in direct contact with the siliconlayer 8 is converted to its silicide and forms buried silicide regions25 (FIG. 22, which are the buried bit lines 25 of the device array 400of FIG. 29). Preferably, the metal capable of forming a silicide is acombination of cobalt/titanium nitride material that forms cobaltsilicide bit line 25. However, the metal silicide may comprise any metalcapable of forming a silicide, including but not limiting to cobalt,nickel, molybdenum, titanium, tungsten, tantalum, and platinum, amongothers, and combinations of such materials. In addition, the metalsilicide may also comprise combinations of silicides doped withnitrogen, such as cobalt nitride silicide, tungsten nitride silicide, ora combination of tungsten nitride silicide/tungsten silicide, forexample.

Subsequent to the formation of buried silicide bit lines 25, theunreacted metal is stripped and a nitride material 26 is formed withinthe first set of trenches 21, as shown in FIG. 22. Although nitridematerial is preferred, the invention also contemplates the formation ofan oxide, such as silicon oxide for example, to fill in the first set oftrenches 21, but must be dissimilar to the material of insulating layer51. The device array 400 is then planarized by any suitable means, suchas chemical mechanical polishing (CMP), stopping at the insulatingcolumns 51 a, and the insulating columns 51 a are subsequently removedby known methods in the art to form the structure of FIG. 23.

FIG. 24 illustrates epitaxial silicon layers 414, 416, 418 formed withintrenches 21 a (FIG. 23) by known methods, for example, by epitaxialgrowth, such as vapor phase, liquid phase, or solid phase epitaxy. Forexample, the first epitaxial silicon layer 414 (FIG. 24) may be grown byepitaxy in a reaction chamber at high temperatures, of about 900-1200°C., and by employing a silicon gas source that introduces a gaseousspecies containing silicon (Si) into the reaction chamber. As known inthe art, the silicon gas source may be silane (SiH₄), higher ordersilanes, such as disilane (Si₂H₆), as well as other gaseous sources ofsilicon, such as dichlorsilane (SiH₂Cl₂), trichlorsilane (SiHCl₃), ortetrachlorsilane (SiCl₄), for example. In any event, the first epitaxialsilicon layer 414 is grown over the SOI substrate 110 to a thickness ofabout 500 to about 3,000 Angstroms, preferably of about 2,000 Angstroms.Subsequent to, or during, the formation of the first epitaxial siliconlayer 414, n-type dopants such as phosphorous (P), arsenic (As) orantimony (Sb) are introduced into the first epitaxial silicon layer 414to form an n-type epitaxial silicon layer 414. A heat treatment, such asan anneal treatment at about 600° C. to about 1000° C., may beoptionally employed to activate the dopant within the n+ epitaxialsilicon layer 414.

Once the growth of the first epitaxial silicon layer 414 is completed,and while the SOI substrate 110 is still in the reaction chamber, asecond epitaxial silicon layer 416 and a third epitaxial silicon layer418 are sequentially formed over the first epitaxial silicon layer 414,as shown in FIG. 24, by methods similar to, or different from, thoseemployed for the formation of the first epitaxial silicon layer 414. Thesecond epitaxial silicon layer 416 is doped with a p-type dopant, suchas boron (B), boron fluoride (BF₂) or indium (In), and is formed to athickness of about 500 to about 2,000 Angstroms. The third epitaxialsilicon layer 418 is doped with an n-type dopant, different from orsimilar to that for the formation of the n-type first epitaxial siliconlayer 414, and is formed to a thickness of about 500 to about 1,500Angstroms, preferably of about 1,000 Angstroms. A heat treatment, forexample, an anneal treatment at about 600° C. to about 1000° C., may beoptionally employed to activate the dopant within each of the second andthird epitaxial silicon layers 416, 418.

Subsequent to the formation of epitaxial silicon layers 414, 416, 418,the formation of word lines (i.e., gate electrodes) 435 (FIG. 28) of thevertical transistors 490 (FIG. 29) proceeds according to a methoddescribed above with reference to the first embodiment and as shown inFIGS. 7-10. As such, FIGS. 25-28 correspond to FIGS. 7-10 of the firstembodiment, and illustrate side-to-side cross-sectional views of thedevice array 400 of FIG. 29, taken along lines A-A and B-B and at aninitial stage of processing, but subsequent to the formation of thesilicide bit lines 25 described above. The illustrations in FIGS. 25-28are cross-sectional views taken normal to the buried bit lines 25 but attwo different locations at the array 400 (A-A) and (B-B).

FIG. 25 (A-A) illustrates gate stack 420 comprising epitaxial siliconlayers 414, 416, 418 after the formation of the silicide bit lines 25 ofFIG. 24. FIG. 26 illustrates the next step in the process, in whichsecond set of trenches 23 (FIG. 25) are filled with an insulatingmaterial 31, preferably an oxide material such as silicon oxide, whichis etched back by known methods in the art to form oxide layer 32, asshown in FIG. 27. The height of the oxide layer 32 is tailored to allowisolation of the already-formed silicide bit lines 25 from theto-be-formed word lines 435. Subsequent to the formation of the oxidelayer 32, a thin gate oxide layer 434 and a gate electrode layer 435 aresequentially formed on the sidewalls of the stacks 420, as shown in FIG.28. The thin gate oxide layer 434, which will act as a gate insulatorlayer, may comprise silicon dioxide (SiO₂), for example, which may bethermally grown in an oxygen ambient, at a temperature between about600° C. to about 1000° C. and to a thickness of about 10 Angstroms toabout 100 Angstroms. The gate insulator is not limited to silicon oxideand other dielectric materials such as oxynitride, Al₂O₃, Ta₂O₅ or otherhigh k material may be used as gate insulator layer.

As illustrated in FIG. 28, gate layer 435 is formed over the thin gateoxide layer 434. The gate layer 435 is formed of doped polysilicon whichmay be formed over the thin gate oxide layer 434 by, for example, a lowplasma chemical vapor deposition (LPCVD) method at a temperature ofabout 300° C. to about 700° C. and to a thickness of about 100 Angstromsto about 2,000 Angstroms. An anisotropic RIE is then used to define thedouble gate electrode 435 orthogonal to the buried silicide bit lines25. Subsequent processing steps are then applied to complete theformation of the device array 400 comprising MOSFET transistor 490. Eachof the vertical transistor 490 of a particular column in the array 400is formed of drain 418 and source 414, with double gate electrode 435formed over the thin gate oxide 34 of each transistor 490. The gateelectrode is vertical and orthogonal to the buried bit line 25. Thevertical gate electrode forms word line 435 which electrically connectsall of the cells 499 of a given row in the array 400. Once again,capacitors are formed over the vertical transistors 490 with oneelectrode in contact with drain 418. Other processing steps are thencarried out to interconnect the word line, bit lines and capacitors ofthe memory cells 499 in a memory array, as described above withreference to the first embodiment.

FIGS. 30-36 illustrate a fifth embodiment of the present invention,according to which epitaxial silicon layers 414, 416, 418 are formed aspart of stack 420 provided over a p-type silicon wafer 210, and not overa SOI substrate, such as the SOI substrate 110 described above. As inthe previously-described embodiment with reference to FIGS. 20-29,epitaxial silicon layers 414, 416, 418 are formed as part of stack 420subsequent to the formation of the silicide bit lines 25 of FIG. 33, andas part of DRAM cells 599 of device array 500 (FIG. 36).

Accordingly, FIG. 30 illustrates the formation of an n+ silicon layer 12and a thick insulating layer 51 of about 2,000 Angstroms to about 10,000Angstroms, which are formed over the p-type silicon wafer 210 bychemical vapor deposition (CVD) or other suitable methods, for example.The n+ silicon layer 12 may be also formed by appropriately doping a topportion of the p-type silicon wafer 210, as explained above withreference to the previously described embodiments. As also described inthe above embodiments, the insulating layer 51 may be formed via PECVDand LPCVD deposition procedures, for example, at a temperature betweenabout 300° C. to about 1000° C.

A photoresist and mask are then applied over the thick insulating layer51, and photolithographic techniques are used to define a set ofparallel oxide rows 51 a on the array surface. A directional etchingprocess such as plasma etching or reactive ion etching (RIE) is used toetch into the n+ silicon layer 12 to form a first set of trenches 21, asdepicted in FIG. 30.

A conductive layer 24 of a metal capable of forming a silicide is formedover insulating columns 51 a and within the first set of trenches 21(FIG. 31) by RF or DC sputtering, or by other similar methods such asCVD, to a thickness of about 100 Angstroms to about 800 Angstroms.Subsequent to the deposition of the metal capable of forming a silicide,the substrate is subjected to a rapid thermal anneal (RTA), typicallyfor about 10 to 60 seconds, using a nitrogen ambient at about 600° C. toabout 850° C., so that the metal in direct contact with the n+ siliconlayer 12 is converted to its silicide and forms buried silicide regions25 (which are the buried bit lines 25 of the device array 500 of FIG.36). The metal capable of forming a silicide may be the same as, ordifferent from, the metals capable of forming silicides described abovewith reference to the formation of the buried silicide regions 25 of theprevious embodiments.

Subsequent to the formation of buried silicide bit lines 25, theunreacted metal is stripped (FIG. 33) and the silicide is etchedtogether with the n+ silicon layer 12 down to the p-type siliconsubstrate 210 and a nitride material 26 is formed within the first setof trenches 21, as shown in FIG. 34. Although nitride material ispreferred, the invention also contemplates the formation of an oxide,such as silicon oxide for example, to fill in the first set of trenches21, but it must be dissimilar to the material of the insulating layer51. The device array 500 is then planarized by any suitable means, suchas chemical mechanical polishing (CMP), for example, stopping at theinsulating columns 51 a.

Subsequently, insulating columns 51 a are removed and epitaxial siliconlayers 414, 416, 418 (FIG. 35) are formed by known methods, for example,by epitaxial growth, such as vapor phase, liquid phase, or solid phaseepitaxy. For example, the first epitaxial silicon layer 414 may be grownby epitaxy in a reaction chamber at high temperatures, of about900-1200° C., and by employing a silicon gas source that introduces agaseous species containing silicon (Si) into the reaction chamber. Asknown in the art, the silicon gas source may be silane (SiH₄), higherorder silanes, such as disilane (Si₂H₆), as well as other gaseoussources of silicon, such as dichlorsilane (SiH₂Cl₂), trichlorsilane(SiHCl₃), or tetrachlorsilane (SiCl₄), for example. In any event, thefirst epitaxial silicon layer 414 is grown over the substrate 210 to athickness of about 500 to about 3,000 Angstroms, preferably of about2,000 Angstroms. Subsequent to, or during, the formation of the firstepitaxial silicon layer 414, n-type dopants such as phosphorous (P),arsenic (As) or antimony (Sb) are introduced into the first epitaxialsilicon layer 414 to form an n-type epitaxial silicon layer 414. A heattreatment, such as an anneal treatment at about 600° C. to about 1000°C., may be optionally employed to activate the dopant within the n+epitaxial silicon layer 414.

Once the growth of the first epitaxial silicon layer 414 is completed,and while the substrate is still in the reaction chamber, a secondepitaxial silicon layer 416 and a third epitaxial silicon layer 418 aresequentially formed over the first epitaxial silicon layer 414, as shownin FIG. 35, by methods similar to, or different from, those employed forthe formation of the first epitaxial silicon layer 414. The secondepitaxial silicon layer 416 is doped with a p-type dopant, such as boron(B), boron fluoride (BF₂) or indium (In), and is formed to a thicknessof about 500 to about 2,000 Angstroms. The third epitaxial silicon layer418 is doped with an n-type dopant, different from or similar to thatfor the formation of the n-type first epitaxial silicon layer 414, andis formed to a thickness of about 500 to about 1,500 Angstroms,preferably of about 1,000 Angstroms. A heat treatment, for example, ananneal treatment at about 600° C. to about 1000° C., may be optionallyemployed to activate the dopant within each of the second and thirdepitaxial silicon layers 416, 418.

Subsequent to the formation of epitaxial silicon layers 414, 416, 418,the formation of gate electrode lines 535 or word lines 535 (FIG. 35) ofthe vertical transistors 590 (FIG. 36) proceeds according to a methodsimilar to that described above with reference to the first embodimentand as shown in FIGS. 7-10. As such, a thin gate oxide layer 534 and agate electrode 535 are sequentially formed on the sidewalls of thestacks 420, as shown in FIG. 35. The thin gate oxide layer 534, whichwill act as a gate insulator layer, may comprise silicon dioxide (SiO₂),for example, which may be thermally grown in an oxygen ambient, at atemperature between about 600° C. to about 1000° C. and to a thicknessof about 10 Angstroms to about 100 Angstroms. The gate insulator is notlimited to silicon oxide and other dielectric materials such asoxynitride, Al₂O₃, Ta₂O₅ or other high k material may be used as gateinsulator layer. Vertical double gate electrode 535 is formed over thethin gate oxide layer 534 and orthogonal to the buried silicide bitlines 25. Subsequent processing steps are then applied to complete theformation of the device array 500 comprising MOSFET transistors 590(FIG. 36). Each of the vertical transistor 590 of a particular column inthe array 500 is formed of drain 418 and source 414, with double gateelectrode 535 formed over the thin gate oxide 534 of each transistor590. The gate electrode is vertical and orthogonal to the buried bitline 25. The vertical gate electrode forms word line 535 whichelectrically connects all of the cells 599 of a given row in the array500. Subsequent processing steps are then carried out to interconnectthe word lines, the bit lines and capacitors of the memory cells 599 ina memory array, as described above with reference to the firstembodiment.

FIG. 37 illustrates yet another embodiment of the present inventionaccording to which the device array 600 comprises MOSFET transistors 690which are subjected to an optional salicide process after the formationof the vertical gate electrode 435, 535 (FIGS. 29 and 36). The MOSFETtransistors 690 may be formed over a SOI substrate (as the SOI substrate110 described above with reference to FIGS. 20-29) or over a p-typesubstrate (as the p+ substrate 210 described above with reference toFIGS. 30-36) and comprises epitaxial silicon layers 414, 416, 418 formedas part of stack 420 and as described in detail above. For illustrationpurposes only, the MOSFET transistors 690 are fabricated as describedabove over a p-type substrate 210, in a manner similar to that for theformation of the MOSFET transistors 590 of FIG. 36. However, subsequentto the metal deposition and the formation of the device array 500 ofFIG. 36, the array 600 is subjected to an anneal process such as a rapidthermal anneal (RTA) for about 10 to 60 seconds using a nitrogen ambientat about 600° C. to about 850° C.) to form gate electrodes 535 of metalsilicides and a silicide layer 685 preferably of cobalt silicide(CoSi₂), or other silicide material, on top of the stack 420. The cobaltsilicide formed on top of the stack 420 may be formed simultaneouslywith the formation of the buried silicide bit lines 25 or,alternatively, after the formation of the buried silicide bit lines 25.Of course, the metal for the formation of the silicide gate electrode535 must be a metal which may be converted to its silicide or acombination of such metals, for example. In this manner, the word line535, the buried bit line 25 and the drain including layer 685 are allformed of a silicide material.

Reference is now made to FIG. 38, which illustrates a partial top viewof the device array 500 of FIG. 36. Squarely shaped layout 550corresponds to the device array 500 of FIG. 36 and is formed of squareunits B. As illustrated in FIG. 38, the area of a single square B₁ ofthe squarely shaped layout 550 is about 4F², where F is the minimumlithographic feature size. Accordingly, the DRAM cells of the devicearray 500 have an area of about 4F² (where F is the minimum lithographicfeature size) that comprises a vertical transistor having at least aburied silicide bit line. As may be readily appreciated by personsskilled in the art, decreasing the size of the DRAM cell to about 4F²(where F is the minimum lithographic feature size) while maintainingcommon processing steps with peripheral devices decreases fabricationcosts while increasing array density. By forming at least a buried bitline, such as the buried bit line 25 described above, and by forming atleast a double gate vertical electrode, such as vertical double gateelectrodes 35, 335, 435, 535, high density and high performance arraysare produced by the fabrication processes of the present invention.

Although the above embodiments have been described with reference to theformation of NMOS vertical transistors having at least a double buriedbit line and at least a double vertical gate electrode, it must beunderstood that the invention is not limited to this embodiment.Accordingly, the invention also contemplates the formation of PMOStransistors, as well as the formation of a plurality of MOS transistorsof the same or different conductivity type. Thus, the above illustratedand described embodiments are only exemplary, and the present inventionis not limited to the illustrated embodiments.

In addition, although the formation of n+, p−, n+ regions or devicelayers 12, 14, 16 has been described above with reference to the dopingof a SOI substrate (such as SOI substrate 110) or of a silicon substrate(such as p-type silicon substrate 210), the invention is not limited tothese embodiments and also contemplates the formation of device layers12, 14, 16 by other known methods in the art. For example, device layers12, 14, 16 may be doped silicon or doped polysilicon layers formed overa SOI substrate or over a silicon substrate, or partially within a SOIsubstrate or a silicon substrate. In these embodiments, the n+, p−, n+regions or device layers 12, 14, 16 may be formed by deposition methods,for example, by CVD, PECVD or LPCVD, among others, or by other knownmethods of the art. Accordingly, the embodiments described above withreference to the formation of n+, p−, n+ regions or device layers 12,14, 16 by doping or implanting predefined regions of a SOI substrate orof a p-type silicon substrate are only exemplary, and the invention isnot limited to these exemplary embodiments.

Further, although the above embodiments have been described withreference to the formation of vertical transistors having at least adouble buried bit line and at least a double vertical gate electrode, itmust be understood that the invention is not limited to theseembodiments. Accordingly, the invention also contemplates the formationof vertical transistors having only one buried bit line formed by theembodiments described above. The invention also contemplates embodimentsin which the buried bit line of the vertical transistor is at leastpartially buried and not completely buried, as described in theembodiments above. Further, the invention does not contemplateembodiments where only pairs of bit lines or pairs of gates are formedassociated with each vertical transistor. Thus, the invention could beadapted for use to form one bit line and one vertical gate pertransistor.

The above description illustrates preferred embodiments that achieve thefeatures and advantages of the present invention. It is not intendedthat the present invention be limited to the illustrated embodiments.Modifications and substitutions to specific process conditions andstructures can be made without departing from the spirit and scope ofthe present invention. Accordingly, the invention is not to beconsidered as being limited by the foregoing description and drawings,but is only limited by the scope of the appended claims.

1-49. (canceled)
 50. A semiconductor memory array comprising: aplurality of memory cells arranged into an array of rows and columns,wherein each memory cell comprises a vertical transistor having first,second and third conductive regions, wherein the third conductive regionof the vertical transistor is electrically coupled to one electrode of acapacitor, at least one of the plurality of memory cells having an areaof about 4F², where F is the minimum lithographic feature size; aplurality of pairs of first and second bit lines located below thevertical transistors, wherein each pair of first and second bit lines isin contact with the first conductive region of each vertical transistorin a respective column; and a plurality of word lines, wherein each wordline adjoins the second conductive region of each vertical transistor ina respective row, and is separated from the second conductive region bya gate insulating layer. 51-53. (canceled)
 54. The memory array of claim50, wherein at least one of the bit lines and at least one of word linesforms an angle of about ninety degrees relative to each other.
 55. Thememory array of claim 50, wherein the first, second and third regions ofthe vertical transistor are doped silicon regions, wherein the first andthe third conductive regions are doped to a second conductivity type,and the second conductive region is doped to a first conductivity type.56. The memory array of claim 50, wherein the first, second and thirdconductive regions of the vertical transistor are doped epitaxialsilicon regions, wherein the first and the third epitaxial regions aredoped to a second conductivity type, and the second epitaxial region isdoped to a first conductivity type.
 57. The memory array of claim 50,wherein the plurality of bit lines comprise a silicide material. 58-91.(canceled)
 92. The memory array of claim 50 wherein the word linescomprises conductively doped semiconductive material.
 93. The memoryarray of claim 50 wherein the memory cells comprise DRAM cells.
 94. Thememory array of claim 50 wherein the bit lines are of differentcomposition from those of the first conductive regions.
 95. The memoryarray of claim 50 wherein the bit lines are not laterally coextensivewith the first conductive regions.
 96. The memory array of claim 50wherein the bit lines are not longitudinally coextensive with the firstconductive regions.
 97. The memory array of claim 50 wherein the bitlines are not laterally and longitudinally coextensive with the firstconductive regions.
 98. A semiconductor memory array comprising: aplurality of memory cells arranged into an array of rows and columns,wherein each memory cell comprises a vertical transistor having first,second and third conductive regions, wherein the third conductive regionof the vertical transistor is electrically coupled to one electrode of acapacitor, at least one of the plurality of memory cells having an areaof about 4F², where F is the minimum lithographic feature size; aplurality of pairs of first and second bit lines located below thevertical transistors, wherein each pair of first and second bit lines isin contact with the first conductive region of each vertical transistorin a respective column; and a plurality of pairs of first and secondword lines, wherein each pair of first and second word line adjoins thesecond conductive region of each vertical transistor in a respectiverow, and is separated from the second conductive region by a gateinsulating layer.
 99. The memory array of claim 98, wherein the first,second and third conductive regions of the vertical transistor are dopedepitaxial silicon regions, wherein the first and the third epitaxialregions are doped to a second conductivity type, and the secondepitaxial region is doped to a first conductivity type.
 100. The memoryarray of claim 98, wherein the plurality of bit lines comprise asilicide material.
 101. The memory array of claim 98 wherein the memorycells comprise DRAM cells.
 102. The memory array of claim 98 wherein thebit lines are of different composition from those of the firstconductive regions.
 103. The memory array of claim 98 wherein the bitlines are not laterally coextensive with the first conductive regions.104. The memory array of claim 98 wherein the bit lines are notlongitudinally coextensive with the first conductive regions.
 105. Thememory array of claim 98 wherein the bit lines are not laterally andlongitudinally coextensive with the first conductive regions.